Electron-hole Pair Production Research Articles

Morphological Dependence of Metal Oxide Photocatalysts for Dye Degradation

There is no doubt that organic dyes currently play an indispensable role in our daily life; they are used in products such as furniture, textiles, and leather accessories. However, the main problems related to the widespread use of these dyes are their toxicity and non-biodegradable nature, which mainly are responsible for various environmental risks and threaten human life. Therefore, the elimination of these toxic materials from aqueous media is highly recommended to save freshwater resources, as well as our health and environment. Heterogeneous photocatalysis is a potential technique for dye degradation, in which a photocatalyst is used to absorb light (UV or visible) and produce electron–hole pairs that enable the reaction participants to undergo chemical changes. In the past, various metal oxides have been successfully applied as promising photocatalysts for the degradation of dyes and various organic pollutants due to their wide bandgap, optical, and electronic properties, in addition to their low cost, high abundance, and chemical stability in aqueous solutions. Various parameters play critical roles in the total performance of the photocatalyst during the photocatalytic degradation of dyes, including morphology, which is a critical factor in the overall degradation process. In our article, the recent progress on the morphological dependence of photocatalysts will be reviewed.

Embedding Atomically Dispersed Manganese/Gadolinium Dual Sites in Oxygen Vacancy-Enriched Biodegradable Bimetallic Silicate Nanoplatform for Potentiating Catalytic Therapy.

Due to their atomically dispersed active centers, single-atom nanozymes (SAzymes) have unparalleled advantages in cancer catalytic therapy. Here, loaded with chlorin e6 (Ce6), a hydrothermally mass-produced bimetallic silicate-based nanoplatforms with atomically dispersed manganese/gadolinium (Mn/Gd) dual sites and oxygen vacancies (OVs) (PMnSA GMSNs-V@Ce6) is constructed for tumor glutathione (GSH)-triggered chemodynamic therapy (CDT) and O2 -alleviated photodynamic therapy. The band gaps of silica are significantly reduced from 2.78 to 1.88eV by doping with metal ions, which enables it to be excited by a 650nm laser to produce electron-hole pairs, thereby facilitating the generation of reactive oxygen species. The Gd sites can modulate the local electrons of the atom-catalyzed Mn sites, which contribute to the generation of superoxide and hydroxyl radicals (• OH). Tumor GSH-triggered Mn2+ release can convert endogenous H2 O2 to • OH and realize GSH-depletion-enhanced CDT. Significantly, the hydrothermally generated OVs can not only capture Mn and Gd atoms to form atomic sites but also can elongate and weaken the O-O bonds of H2 O2 , thereby improving the efficacy of Fenton reactions. The degraded Mn2+ /Gd3+ ions can be used as tumor-specific magnetic resonance imaging contrast agents. All the experimental results demonstrate the great potential of PMnSA GMSNs-V@Ce6 as cancer theranostic agent.

Impact ionization in silicon at low charge-carrier energies

Photons absorbed in silicon produce electron–hole pairs, which can cause impact ionization and quantum yield larger than one. Reliable determination of quantum yield at low charge-carrier energies (<4 eV) has been challenging because photon losses due to reflectance and charge-carrier losses due to recombination affect the resulting photocurrent. Here, we present how the measurement of this fundamental characteristic of silicon crystals can be improved in the charge-carrier energy range of 1.6–4 eV by using a predictable quantum efficient detector based on induced junction photodiodes optimized for photon-to-electron conversion efficiency. The measured quantum yield values are compared with the results of theoretical calculations, revealing increased impact-ionization probabilities at 2.25 and 3.23 eV on the top of a smooth background curve calculated by a model based on free charge carriers in the silicon lattice. For the results at the lowest energies, both data and an asymptotic extrapolation model suggest that quantum yield exceeds unity by ∼10−4 at 1.6 eV corresponding to a photon wavelength of 450 nm.

Simulation of Electron-Hole Pairs Created by γ-Rays Interaction in Scintillation Crystals Using Geant4 Toolkit

Understanding the electron-hole pair (e-h pair) production process in scintillators is essential for further studies of the scintillation mechanism. The number of e-h pairs can be derived from theoretical and empirical approaches. However, simulation approaches have been chosen to study all the related physical processes and spatial information in scintillation crystals. Due to the complexity of particle interaction and physical properties of solid-state, the simulation approach could be better than the analytical approach. This work aims to develop a Geant4-based simulation toolkit for studying scintillation mechanisms in single-crystalline scintillators since Geant4 can accurately calculate particle energy loss. Due to the lack of a solid-state physic model for insulators, the plasmon creation and phonon loss process for electrons are implemented in the Geant4 toolkit. The electrons, which are created by γ-rays interaction in crystals, can make further ionization to generate secondary electrons or create plasmons, or lose energy to phonons. Then the created plasmons decay into e-h pairs. These electrons and holes are tracked with information on energy and position. Therefore, the number of created e-h pairs and their spatial distribution can be extracted. In this work, we will present the result of the creation process of e-h pairs per MeV in scintillation crystals induced by γ-rays interaction. The number of e-h pairs created by γ ray in some popular inorganic crystals has been calculated. Assuming maximum conversion efficiency from the e-h pair to the luminescence light, we can calculate the maximum light yield of the intrinsic scintillator.

Optimization of 3×3 inch NaI(Tl) detector related to energy, distance and bias voltage

There are different types of radiation detectors (gas-filled, semiconductor, scintillator etc.) and various working principles (ionization, electron-hole pair production, excitation) used in radiation detection. It is important that any detector is selected according to its intended use and that the detector is operated under appropriate conditions. NaI(Tl) scintillation detectors are widely used in nuclear spectroscopic studies due to their known advantages. Prior to carrying out such studies, it is first necessary to know the response function of the detector against radiation under the expected operating conditions and to perform the appropriate calibrations. It is important that certain factors are optimized, such as the experimental setup (source-to-detector position, shielding, source activity etc.), gain, bias voltage, and shaping time, which can each significantly affect the response function. In addition, in computational studies based on experimentation, optimization is gain more significant since it is assumed that such experimental studies are carried out under optimum conditions. In this study, in order to determine the most suitable operating conditions for a 3 × 3 inch NaI(Tl) detector; a coaxial sample holder was manufactured to enable different source-to-detector distances (5–15 cm) to be tested, and spectra were then obtained using different bias voltages (750–850 V) and sources with different energies in the range of 81–1332 keV. Through the use of efficiency values obtained for each condition, line and 3D surface plots were drawn and the relations between efficiency and distance, energy and bias voltage examined. The results showed that the efficiency value had an exponentially decreasing characteristic that was dependent on the energy and source-detector distance, and had the highest value at a bias voltage of around 800 V.

Unruh effect and Takagi's statistics inversion in strained graphene

We present a theoretical study of how a spatially varying quasiparticle velocity in honeycomb lattices, achievable using strained graphene or in engineered cold-atom optical lattices that have a spatial dependence to the local tunneling amplitude, can yield the Rindler Hamiltonian embodying an observer accelerating in Minkowski space-time. Within this setup, a sudden switch on of the spatially varying tunneling (or strain) yields a spontaneous production of electron-hole pairs, an analog version of the Unruh effect characterized by the Unruh temperature. We discuss how this thermal behavior, along with Takagi's statistics inversion, can manifest themselves in photoemission and scanning tunneling microscopy experiments. We also calculate the average electronic conductivity and find that it grows linearly with frequency $\ensuremath{\omega}$. Finally, we find that the total system energy at zero environment temperature looks like Planck's blackbody result for photons due to the aforementioned statistics inversion, whereas for an initial thermally excited state of fermions, the total internal energy undergoes stimulated particle reduction.

NIR to intense UV-Blue photon transition by upconverting LuF3: Yb3+, Tm3+@BiOCl core@shell composite for pollutant degradation

Combining lanthanide ion (Ln3+)-doped upconversion luminescent materials with photocatalysts is an effective strategy to develop near-infrared (NIR)-responsive photocatalysts. Herein, a novel core-shell structured upconversion photocatalyst LuF3: Yb3+, Tm3+@BiOCl (LYT@BiOCl) was synthesized by hydrothermal/solvothermal method. The core is constituted of LuF3: Yb3+, Tm3+ (LYT) upconversion nanoparticles (UCNPs). A BiOCl shell with relatively narrow band-gap was coated on LYT core via a solvothermal process. Solar NIR photons were successfully converted to UV-Blue photons by LYT, which activated the BiOCl shell to produce electron-hole pairs. The excellent core-shell structure of LYT@BiOCl was confirmed by a series of test characterizations, which provides the necessary conditions for efficient energy transfer between LYT and BiOCl. The fluorescence lifetime decay of the 1G4 levels of Tm3+ in LYT@BiOCl further demonstrates the existence of an efficient Förster resonance energy transfer (FRET) channel between LYT and BiOCl. Moreover, photocurrent and EIS tests illustrated LYT(10)@BiOCl has greater photocurrent response and smaller interface transfer resistance. As a result, under the light irradiation of λ > 800 nm and λ > 400 nm (300 W Xenon lamp with a filter), the photocatalytic performance of LYT@BiOCl was always higher than pure BiOCl. Especially, a great enhancement of + 42% and + 64% on the MB degradation efficiency was achieved in LYT(10)@BiOCl compared with pure BiOCl, respectively. This work might provide a useful reference for the rational design of highly efficient NIR-responsive photocatalysts.

CuO decorated graphene TiO2 derived MIL-125 nanocomposite with enhanced photo-response as a highly efficient indirect sunlight driven photocatalyst

Advanced Oxidation Processes (AOPs) are utilized to remove a large range of pollution in water by heterogeneous photocatalysis with production of electron-hole pairs by light. Herein, we showed the preparation of GO/MIL-125 by a simple solvothermal method and then the addition CuO by a simple wet impregnation method. MIL-125 and their derived have attracted considerable attraction due to high earth’s crust abundance, low toxicity, unique photo redox property and photocatalytic application. The photocatalyst, 5CuO/10GO/MIl-125 which was calcined at 600 ◦C (CuTG-600), with a bandgap of 2.82 eV, was utilized to degrade Rhodamine B (RhB) dye which is oxidized to CO2 and H2O. The characterization of different samples was investigated using FT-IR, XRD, XPS, SEM, EDX mapping, UV–vis spectroscopy, Linear sweep voltammetry, photocurrent response, electrochemical impedance spectroscopy (EIS) and Mott-Schottky plots. The combination of GO and CuO increases the degradation activity of RhB dye under irradiation of visible light (λ = 380–780), (blue LED lamp (λ = 450 nm) and red LED lamp (λ = 610 nm)) and the higher results were showed by 5CuTG-600. Photodegradation kinetics were explained by various parameters such as calcination, catalyst type, pH, dye concentration, catalyst dose, different lamps with different wavelengths, different scavengers and reusability. Furthermore, The Photoelectrochemical test results were noticed that the activity of 5CuTG-600 is higher than that of TG-600 and T-600 due to higher donor concentration (ND), and lower recombination process of the charge carrier. Finally, we obtain 5CuTG-600 which acts as an important photocatalyst and is used in environmental and electrochemical applications.

Recent Progress of Single-Atom Photocatalysts Applied in Energy Conversion and Environmental Protection.

Photocatalysis driven by solar energy is a feasible strategy to alleviate energy crises and environmental problems. In recent years, significant progress has been made in developing advanced photocatalysts for efficient solar-to-chemical energy conversion. Single-atom catalysts have the advantages of highly dispersed active sites, maximum atomic utilization, unique coordination environment, and electronic structure, which have become a research hotspot in heterogeneous photocatalysis. This paper introduces the potential supports, preparation, and characterization methods of single-atom photocatalysts in detail. Subsequently, the fascinating effects of single-atom photocatalysts on three critical steps of photocatalysis (the absorption of incident light to produce electron-hole pairs, carrier separation and migration, and interface reactions) are analyzed. At the same time, the applications of single-atom photocatalysts in energy conversion and environmental protection (CO2 reduction, water splitting, N2 fixation, organic macromolecule reforming, air pollutant removal, and water pollutant degradation) are systematically summarized. Finally, the opportunities and challenges of single-atom catalysts in heterogeneous photocatalysis are discussed. It is hoped that this work can provide insights into the design, synthesis, and application of single-atom photocatalysts and promote the development of high-performance photocatalytic systems.

In situ fabrication of co-coordinated TCPP-Cur donor-acceptor-type covalent organic framework-like photocatalytic hydrogel for rapid therapy of bacteria-infected wounds

Pathogenic bacterial infection and the presence of drug-resistant bacteria have been threatening the health of human. Phototherapy has attracted increasing attention for treating tissue infections because it is noninvasive and does not lead to drug resistance. Herein, we construct a Cu co-coordinated donor-acceptor type (D-A) covalent organic framework-like (COF-like) and sodium alginate (SA) hydrogel (CTCS hydrogel) with photocatalytic and anti-inflammatory properties for the treatment of wound infection. The COF-like (TCPP-Cur) can be photoexcited with 660 nm light to produce electron-hole pairs that would be separated by donor-acceptor pairs. The co-coordinated Cu can reduce the band gap of TCPP-Cur, increase the separation rate of photogenerated electron-hole pairs, and improve the photocatalytic performance of CTCS hydrogel. Under 660 nm irradiation, the CTCS hydrogel can achieve 99.95% and 98.5% bactericidal activity against Staphylococcus aureus (S. aureus) and Escherichia coli (E. coli) within 20 min owing to the synergistic interactions of the photothermal and photocatalytic effects. In vivo tests have shown that CTCS hydrogel can reduce the expression of TNF-α and enhance the expression of IL-10 and VEGF to promote wound healing and tissue regeneration. This photocatalytic hydrogel provides a promising strategy for reconstructing bacterially infected tissues rapidly.

Strain Engineering of ZrO2@TiO2 Core@shell Nanoparticle Photocatalysts

TiO2 photocatalysts can provide carbon-capture utilization and storage by converting atmospheric CO2 to green hydrogen, but the efficiency of the current photocatalysts is still too low for economical usage. Anatase TiO2 is effective in transferring the electrons and holes produced by the photoelectric effect to reactants because of its oxygen-terminated surfaces. However, the anatase TiO2 bandgap is 3.2 eV, which requires photons with wavelengths of 375 nm or less to produce electron–hole pairs. Therefore, TiO2 is limited to using a small part of the solar spectrum. Strain engineering has been used to design ZrO2@TiO2 core@shell structures with large strains in the TiO2 shell, which reduces its bandgap but maintains octahedral facets for charge separation and oxygen-terminated surfaces for the catalysis of reactants. Finite element analysis shows that shell thicknesses of 4–12 nm are effective at obtaining large strains in a large portion of the shell, with the largest strains occurring next to the ZrO2 surface. The c-axis strains for 4–12 nm shells are up to 7%. The strains reduce the bandgap in anatase TiO2 up to 0.35 eV, which allows for the use of sunlight with wavelengths up to 421 nm. For the AM 1.5 standard spectrum, electron–hole pair creation in 4 nm thick and 10 nm thick TiO2 shells can be increased by a predicted 25% and 23%, respectively. The 10 nm thick shells provide a much larger volume of TiO2 and use proportionally less ZrO2. In addition, surface-plasmon resonators could be added to further extend the usable spectrum and increase the production of electron–hole pairs many-fold.

Unveiling quaternary GeSbSeEr chalcogenides as photocatalyst: Degradation of cationic and anionic pollutant in visible light

This article presents the photo-catalytic dye-degradation from pure and doped multi-component Ge17Sb8Se75-xErx(x = 0, 0.2, 0.4, 0.6, 0.8, 1) chalcogenides system has been established in visible light (orange ∼590 nm) for the first time. Band-gap shifting, suitable band position and higher absorption of visible range radiation have been accounted for Er doping to realize the improvement in electron-hole pair production. The improvement in the photo-catalytic activity dye degradation efficiency of 89% in 90 min for methylene blue and 76% for methyl orange has been achieved for certain compositions of chalcogenides glass which were synthesized by traditional melt quench approach. Further, SEM-EDX Photoluminescence and Raman spectroscopy measurements of these samples are supplemented to deliberate the composition, band edge transitions and chemical signatures of Ge17Sb8Se75-xErx. Overall, the presence of Er ions as well as the band to band transition (∼664 nm) in the chalcogenides system is found to play a decisive role in the photo catalytically process involved for competitive dye degradation to replace conventional dyes working in the UV range.

Efficient photoelectrochemical water splitting of metal-porphyrin decorated on BiVO4 photoanode

Decorating sensitizers with excellent light harvesting ability such as porphyrins onto semiconducting materials should be a promising photocatalyst design protocol for prompting electron injection and charge separation at photoelectrolytic interfaces. Here we rationally construct three composite catalysts consisting of BiVO4 and metalloporphyrins (MTPP, M = Co, Ni and Cu) with different metal ion species, nominated as BiVO4/MTPP. The construction of BiVO4/MTPP composites with enhanced light absorption is confirmed by a series of spectroscopic techniques. Under AM 1.5G light irradiation, the composites' potential as photocatalysts for water splitting is investigated. It is found that the photoelectrochemical performance is dependent on the type of central metal ion in the porphyrin core. Among the three BiVO4/MTPP composites, the superior BiVO4/MTPP exhibits the highest photocurrent density of 3.9 mA cm−2 at 1.23 V vs. RHE, which is thought to be due to enhanced electron-hole pair production and a slower recombination rate. Our finding demonstrates a successful porphyrin-doped photocatalyst and highlights the importance of metal type in metalloporphyrins for photoelectrochemical water splitting applications.

Plasmonic Effects of Au@Ag Nanoparticles in Buffer and Active Layers of Polymer Solar Cells for Efficiency Enhancement.

Embedding nanoparticles (NPs) in the buffer layer of bulk heterojunction polymer solar cells (BHJ PSCs) excites the surface plasmonic polaritons and enhances the pathlength of the light in the solar cells. On the other hand, embedding NPs in the active layer significantly improves absorption and increases the production of electron-hole (e-h) pairs in BHJ PSCs. Increasing the volume ratio of NPs embedded in BHJ PSCs enables the direct interfacing of the NPs with the active layer, which then serves as a charge recombination center. Therefore, this study integrates the aforementioned phenomena by exploiting the effects of embedding plasmonic Au@Ag NPs in the buffer and active layers of PSC and then determining the optimum volume ratio of Au@Ag NPs. The results show the absorption is increased across the 350–750 nm wavelength region, and the PCE of the device with embedded Au@Ag in two locations is enhanced from 2.50 to 4.24%, which implies a 69.6% improvement in the PCE in comparison to the reference cell. This improvement is contributed by the combined localized surface plasmon resonance (LSPR) effects of multi-positional Au@Ag NPs, spiky durian-shaped morphology of Au@Ag NPs, and optimized volume ratio of Au@Ag NPs embedded in the PEDOT: PSS and PTB7:PC71BM layers.

Multiphoton absorption and Rabi oscillations in armchair graphene nanoribbons

We present an analytical approach to the problem of the multiphoton absorption and Rabi oscillations in an armchair graphene nanoribbon (AGNR) in the presence of a time-oscillating strong electric field induced by a light wave directed parallel to the ribbon axis. The two-dimensional Dirac equation for the massless electron subject to the ribbon confinement is employed. In the resonant approximation the electron-hole pair production rate, associated with the electron transitions between the valence and conduction size-quantized subbands, the corresponding multiphoton absorption coefficient and the frequency of the Rabi oscillations are obtained in an explicit form. We trace the dependencies of the above quantities on the ribbon width and electric field strength for both the multiphoton assisted and tunneling regimes relevant to the time-oscillating and practically constant electric field, respectively. A significant enhancement effect of the oscillating character of the electric field on the intersubband transitions is encountered. Our analytical results are in qualitative agreement with those obtained for the graphene layer by numerical methods. Estimates of the expected experimental values for the typically employed AGNR and laser parameters show that both the Rabi oscillations and multiphoton absorption are accessible in the laboratory. The data relevant to the intersubband tunneling makes the AGNR a 1D condensed matter analog in which the quantum electrodynamic vacuum decay can be detected by applying an external laboratory electric field.

Enhanced visible-light photovoltaic and photocatalytic performances of SnSe1-xSx nanostructures

In this paper, the pristine SnSe and SnSe1 − xSx nanostructures with different concentrations of sulfur were synthesized by a simple, fast, environmentally-friendly, and inexpensive co-precipitation method. Then, the effect of sulfur concentration on the photovoltaic and photocatalytic properties of the nanostructures was investigated. The results show that in presence of sulfur and its concentration change, the crystallite size and the strain of nanostructures change. Also, among nanostructures, S(6%)-doped SnSe nanostructure indicates the significant photocatalytic activity in the degradation of organic dyes and the significant photovoltaic response under visible light. These results can be due to the reducing crystal defects, the reducing electron-hole recombination, and the improving textural characteristics, which lead to reducing the agglomeration of nanostructures and thus increase the absorption of optical photons and more production of electron-hole pairs. Also, the study of photovoltaic properties shows that the highest photocurrent density and lowest diode coefficient belong to S(6%)-doped SnSe nanostructure.

Kinetic Monte Carlo simulation of hydrogen production from photocatalytic water splitting in the presence of methanol by 1 wt% Au/TiO2

In this research, using the kinetic Monte Carlo simulation (KMC), the hydrogen production from a water-methanol mixture using Au/TiO2 photocatalyst is investigated. A mechanism is proposed, and the rate constants of the reaction steps are specified. The reaction rate constants of different steps and the concentration of the active sites on the photocatalyst surface were determined. An excellent match between simulated and experimental data confirms the results. The electron-hole pair production, methanol adsorption on the photocatalyst surface, and electron-hole recombination steps are considered the most critical steps. To study the effects of independent variables (initial concentration of methanol, photocatalyst dosage, pH, and time of reaction) on the produced hydrogen, a combination of KMC simulation and design of experiment was employed. The concentration of photocatalysis has the highest and pH has the lowest effect on the hydrogen production. The optimal conditions for photocatalytic hydrogen production are presented.

The electrochemical energy storage and photocatalytic performances analysis of rare earth metal (Tb and Y) doped SnO2@CuS composites

The rational design of composite electrode and catalyst materials is critical for increasing high energy density and structural stability, electron-hole pair production in supercapacitors, and photocatalytic applications. By employing a simple hydrothermal technique, we were able to produce SnO2, CuS, SnO2@CuS, Tb doped SnO2@CuS, and Y doped SnO2@CuS samples. Various analytical techniques such as PXRD, FTIR, UV, SEM, EDS, and XPS examine the synthesized materials' structural, optical, purity, morphology, and binding energy. The fabricated asymmetric Y doped SnO2@CuS/AC device revealed the energy and power densities of 16.2 W h kg−1 and 740 W kg−1 at a current density of 1 A/g, with outstanding cycle stability of 80.9% retention capacitance after 5000 charge–discharge cycles. Furthermore, when exposed to visible light, the Y doped SnO2@CuS catalyst degrades the methylene blue dye with a maximum degradation efficiency of 97%. All of the above results suggested that the obtained Y doped SnO2@CuS material can be utilized in supercapacitor and photocatalytic applications.

Natural Negative-Refractive-Index Materials.

Our calculation shows that negative refractive index (NRI), which was known to exist only in metamaterials in the past, can be found in Dirac semimetals (DSM). Electrons in DSM have zero effective mass and hence the system carries no nominal energy scale. Therefore, unlike those of ordinary materials, the electromagnetic responses of the electrons in DSM will not be overwhelmed by the physical effects related to electron mass. NRI is induced by the combination of the quantum effect of vacuum polarization and its finite temperature correction, which is proportional to T^{4} at low temperature. It is a phenomenon of resonance between the incident light and the unique structure of Dirac cones, which allows numerous states to participate in electron-hole pair production excited by the incident light with a similar dispersion relation to that of Dirac cones. The NRI phenomenon of DSM manifests in an extensive range of photon frequencies and wave numbers and can be observed around the gigahertz range at room temperature.

Simulating 50 keV X-ray Photon Detection in Silicon with a Down-Conversion Layer

Simulation results are presented that explore an innovative, new design for X-ray detection in the 20–50 keV range that is an alternative to traditional direct and indirect detection methods. Typical indirect detection using a scintillator must trade-off between absorption efficiency and spatial resolution. With a high-Z layer that down-converts incident photons on top of a silicon detector, this design has increased absorption efficiency without sacrificing spatial resolution. Simulation results elucidate the relationship between the thickness of each layer and the number of photoelectrons generated. Further, the physics behind the production of electron-hole pairs in the silicon layer is studied via a second model to shed more light on the detector’s functionality. Together, the two models provide a greater understanding of this detector and reveal the potential of this novel form of X-ray detection.